The YPG, FSA, US collaboration and "double standards"
An increasingly popular point being made on the part of pro-FSA Tweeters is that anti-imperialists are hypocrites for embracing the YPG while maligning the FSA, despite the fact that both forces collaborate and receive aid from US forces in the region. This argument is intellectually dishonest for the following two reasons:
1.) Anti-imperialist commenters have expressed cynicism about the US' assistance to the YPG by pointing out that it is only being done to co-opt the Syrian Kurds into the anti-Assad cause . Indeed, Reuters reported a year ago that before granting them aid to fight ISIS the Western powers sought to "clarify [the Syrian Kurds'] relationship to President Bashar al-Assad" . It is also perfectly reasonable to assume that the conditions attached to Western help would further compromise the PYD's stated commitment to a progressive societal structure. One can't help but be reminded of the US intervention in Haiti in the 1990s, in which the US military "restored" to power the progressive, democratically elected president Jean-Bertrand Aristide to power on the condition he abide by certain neo-liberal reforms and allow rightist thugs to be integrated into the country's security forces . It should be remembered that the US was behind much of Haiti's right-wing unrest  and exploited the situation to get more progressive elements in line. There is a clear precedent for creating a proxy force and then using it to extract concessions while claiming to oppose said proxy force.
2.) The FSA is represented abroad by liberal and neoconservative expats with deep connections to both the "soft power" USAID/NED/NGO complex and the "hard power" Western defense and intelligence establishment . In Syria itself, it is largely composed of reactionary Islamists who receive massive amounts of aid from the US, the Gulf States and Turkey . While there are undoubtedly some activists and even armed rebels fighting Assad's forces with noble goals in mind, it is increasingly difficult to find progressive elements at the forefront of the actual fighting. In addition to being reactionary, the FSA's end goal of regime change in Syria serves US (and Israeli) goals by removing a relatively independent, militarily strong Arab government from power .
TL;DR: The US collaborates with the YPG because it hopes to co-opt it against Assad and possibly water down its progressive ideology. The US collaborates with the FSA because it is a somewhat reliable proxy army against a counter-hegemonic regime. Thus, the YPG can be supported by anti-imperialists in so far as it remains independent from US influence and designs while the FSA serves US power through-and-through.
 As'ad AbuKhalil, "Who will win in Kobane (`Ayn Al-`Arab)?" 19 October 2014.
 Tom Perry, "West widens contacts with Syria's Kurds but suspicion remains," Reuters, 8 September 2014.
 For US coercion of Aristide into accepting neo-liberal adjustments see:
William Blum, "Haiti, 1986-1994: Who will rid me of this turbulent priest?" in Killing Hope, 2004.
For US collaboration with right-wing paramilitaries while occupying Haiti see:
Allan Nairn, "Haiti under the gun: How US Intelligence has been excercising crowd control," The Nation, 8 Jan 1996.
Also recommended is this entire Twitter thread I made:
 Blum, "Haiti, 1986-1994."
Tim Weiner, "Key Haiti leaders said to have been in the C.I.A.'s pay," New York Times, 1 November 1993.
Allan Nairn, "Occupation Haiti: The eagle is landing," Nation, 3 Oct 1994.
Allan Nairn, "Our man in FRAPH: Behind Haiti's paramilitaries," Nation, 24 Oct 1994.
 Charlie Skelton, "The Syrian opposition: who's doing the talking?" Guardian, 12 July 2012.
 David Mizner, Don’t blame Islam: Al-Qaeda and ISIS are products of US and Saudi imperialism," Jacobin, 30 January 2005.
 An early draft of a Pentagon planning document in the early 1990s spilled the beans on the US' desire to "maintain the mechanisms for deterring potential competitors from even aspiring to a larger regional or global role," quoted in:
Patrick Tyler, "U.S. strategy plan calls for insuring no rivals develop," New York Times, 8 March 2015.
Recently, Israeli figures have been much more open in expressing the benefits of an Assad-free Middle East:
Gilad Sharon, "Who needs Bashar Assad?" YNet News, 12 May 2015.
For evidence that Israel desires the Balkanization of its surrounding Arab states:
Israel Shahak, The Zionist Plan for the Middle East, Association of Arab-American University Graduates, 1982.